The present invention relates generally to the field of wireless telecommunications, and more specifically, to the field of mobile station closed loop power control under weak signal conditions in wireless telecommunications systems, such as code division multiple access (CDMA) wireless telephone systems.
One of the primary standard specifications relevant to the present invention is TIA/EIA/IS-95-A "Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System." This CDMA industry standard specification is considered understood by those reasonably skilled in the art of the present invention. The invention disclosed herein is applicable to other variants of CDMA technology, other than TIA/EIA/IS-95-A.
In CDMA wireless telephone systems (including cellular systems, personal communications systems, satellite communications systems, etc.) power control is crucial to the functionality of the communications system. The available number of simultaneous connections is a function of the transmitted power of the mobile and base stations in the CDMA system. If the mobile and base stations are transmitting at power levels higher than necessary, the overall noise level in the system in increased due to interference between the transmitted signals, thus reducing system capacity. If the mobile and base stations are transmitting power levels lower than necessary, the performance of the CDMA system will be sub-par. Also, since the distance, therefore the received power, between the different mobile stations and the base station will be different, each mobile station and base station pair will be transmitting at a different power level. This implies that transmitted power control has to be performed on an individual basis.
There are two different types of power control, open loop power control and closed loop power control. Open loop power control in a mobile station is a gradual system which dictates that output power should increase as the strength of received signals from a base station decreases, and output power should decrease as received signal strengths increase. As specified in the TIA/EIA/IS-95-A standard, the closed loop power control is performed with the assistance of the base station. The base station measures the signal to noise ratio (SNR) of the signals received from a mobile station and makes a comparison of the measured SNR with a predetermined threshold. Depending on the result of the comparison, the base station will notify the mobile station via power control commands injected into the forward traffic channel to either increase or decrease its transmitted power.
In conditions with low received signal strength, i.e., when the mobile station is far from the base station or there is some large physical body blocking the reception of the transmitted signal, the simple injection of power control commands into the forward traffic channel is not a reliable method of transmitting control information to the mobile station because the injected power control commands are not encoded as is all other data in the CDMA system. In fact, in low received signal strength situations, the mobile station's version of the received power control command may be erroneous and may not represent the true intent of the base station. This can result in the mobile station erratically changing its power output due to unreliable power control data.
There is, therefore, a need in the industry for a system for addressing these and other related and unrelated problems.